Priority measurement rules for channel measurement occasions

ABSTRACT

Based on the determined signal strength of a serving cell, a choice is made whether to utilize a cell-FACH measurement occasion for a lower priority neighbor cell search or for a higher priority neighbor cell search. The lower priority neighbor cell search is for maintaining wireless connectivity and the higher priority neighbor cell search is for accessing enhanced wireless service. In specific examples the search may be inter-frequency for frequency layers with higher or lower priority than a serving layer of the serving cell, or the search may be inter-RAT such as the lower priority GERAN search and a higher priority E-UTRAN search if the serving cell is WCDMA. In one example there are two thresholds for the signal strength, and if higher than both then the measurement occasion is used for an E-UTRAN search regardless of whether GERAN or inter-frequency neighbour cells have been configured for the UE practicing the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/958,528, filed Apr. 20, 2018, which is a continuation of U.S. patentapplication Ser. No. 14/882,188, filed Oct. 13, 2015, which is acontinuation of U.S. patent application Ser. No. 13/023,675, filed Feb.9, 2011, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The exemplary and non-limiting embodiments of this invention relategenerally to wireless communication systems, methods, devices andcomputer programs and, more specifically, relate to inter-frequency andinter-radio access technology measurements made by a user equipment.

BACKGROUND

The following abbreviations that may be found in the specificationand/or the drawing figures are defined as follows:

3GPP third generation partnership progject BSIC base stationidentification code DL downlink E-UTRAN evolved UTRAN (LTE) FACH forwardaccess channel GERAN GSM-enhanced data rates for global evolution (EDGE)GSM global system for mobile communications HSPA high speed packetaccess LTE long term evolution RAT radio access technology TDD timedivision duplex UE user equipment UL uplink UTRAN universal terrestrialradio access network WCDMA wideband code division multiple access

The exemplary embodiments detailed herein are in the context of theWCDMA and HSPA (GSM) wireless systems to resolve problems in measuringinter-frequency and inter-RAT neighbor cells. These teachings are notlimited only to those wireless systems but are more generallyapplicable; the examples merely illustrate specific implementationdetails relevant to those systems.

In the WCDMA/HSPA system the UE can make these inter-frequency andinter-RAT neighbor cell measurements when in the FACH state, which iswhen the UE is camped on a cell and has a signalling connectionestablished with the network. The UE makes such measurements only duringwhat is termed a measurement occasion. Currently, the inter-RATmeasurement occasions are specified for GERAN only, but as E-UTRANbecomes more ubiquitous these neighbor cells are expected to be measuredby the UE camped in the WCDMA/HSPA system also. A problem arises whenincreasing the number of measurement occasions to include E-UTRAN.

Generally, measurement occasions are infrequent and are shared equallybetween all the measurement types configured for the UE, so theeffectiveness of these measurement gaps is quite poor. When E-UTRAN isintroduced for the inter-RAT measurements the effectiveness may becomeeven worse if the current measurement occasion concepts are simplyextended to include E-UTRAN neighbors.

First, consider the current measurement occasion practice which is setforth at 3GPP TS 25.133. The measurement repetition T_(meas) inmilliseconds (ms) is determined by the following algorithm:

T _(meas)=[(N _(FDD) +N _(TDD) +N _(GSM))·N _(TTI) ·M_REP·10];

-   -   where:    -   M_REP is the measurement occasion cycle length where K is given        in Table 8.10A of 3GPP TS 25.133 (K is the FACH measurement        occasion length coefficient, which is specified in 3GPP TS        25.331)

The FACH measurement occasion of N_(TTI) frames will be repeated everyN_(TTI)*M_REP frame. This means that the measurement time T_(meas)increases uniformly for each RAT supported, which has a detrimentalimpact on inter-frequency and inter-RAT measurements and therefore UEmobility. Since currently only GERAN neighbor cells account for theinter-RAT measurements this has not yet become a problem in practice.

To quantify the impact of adding E-UTRAN cells to the inter-RATmeasurements, consider a typical FACH configuration as follows:inter-frequency (N_(FDD)=1); inter-RAT (GERAN) (N_(GSM)=1), where K is 3(MREP=8) and N_(TTI)=1. In this configuration,T_(meas)=(1+0+1)*1*8*10=160 ms.

In this scenario there is an inter-frequency measurement occasion every160 ms, but since it takes about five measurement occasions to perform asearch then there can be a search only every 800 ms. Also in thisscenario a GERAN (inter-RAT) measurement occasion is also configuredevery 160 ms, which as seen at FIG. 1A yields a BSIC verification timeof 7.68 seconds and at FIG. 1B a BSIC refresh time of 6.4 seconds.

Now extend this same measurement occasion protocol to include thepossibility of E-UTRAN neighbor cells. In this straightforward extensionthe measurement time T_(meas) in milliseconds is then defined as:

T _(meas)=[(N _(FDD) +N _(TDD) +N _(GSM) +N _(EUTRA))N _(TTM)_REP10]

Using the same FACH configuration as above thenT_(meas)=(1+0+1+1)*1*8*10=240 ms.

There is therefore an inter-RAT measurement occasion for E-UTRAN every240 ms, but in this case it takes as few as one measurement occasion toperform an E-UTRAN search so there is a search every 240 ms. This alsoprovides an inter-frequency measurement occasion every 240 ms, and sinceit still will take about five measurement occasions to perform a searchthen there can be an inter-frequency search only every 1200 ms.

The inter-frequency measurements would be impacted by including E-UTRANbecause the number of cell-FACH measurement occasions is reduced by athird. This also results in a GERAN measurement every 240 ms, whichresults in a BSIC verification time of 29.76 seconds as seen at FIG. 2A,and a BSIC refresh time of 17.28 seconds as seen at FIG. 2B. This isseen to be too long of a time for GERAN measurements. The teachingsbelow address this problem, but as indicated have utility beyond onlythe GSM/GERAN/E-UTRAN systems which are used only for specificillustration of the principles.

SUMMARY

In a first exemplary embodiment of the invention there is an apparatuscomprising at least one processor and at least one memory storing acomputer program. In this embodiment the at least one memory with thecomputer program is configured with the at least one processor to causethe apparatus to at least: determine signal strength of a serving cell;and choose, based on the determined signal strength, whether to utilizea measurement occasion for a lower priority neighbor cell search or fora higher priority neighbor cell search. In this embodiment the lowerpriority neighbor cell search is for maintaining wireless connectivityand the higher priority neighbor cell search is for accessing enhancedwireless service as compared to the serving cell.

In a second exemplary embodiment of the invention there is a methodcomprising: determining signal strength of a serving cell; and choosing,based on the determined signal strength, whether to utilize ameasurement occasion for a lower priority neighbor cell search or for ahigher priority neighbor cell search. In this embodiment the lowerpriority neighbor cell search is for maintaining wireless connectivityand the higher priority neighbor cell search is for accessing enhancedwireless service as compared to the serving cell.

In a third exemplary embodiment of the invention there is a computerreadable memory storing a computer program, in which the computerprogram comprises: code for determining signal strength of a servingcell; and code for choosing, based on the determined signal strength,whether to utilize a measurement occasion for a lower priority neighborcell search or for a higher priority neighbor cell search. In thisembodiment the lower priority neighbor cell search is for maintainingwireless connectivity and the higher priority neighbor cell search isfor accessing enhanced wireless service as compared to the serving cell.

By example, the measurement occasion in any of the above exemplaryembodiments may be a cell-FACH measurement occasion. These and otherembodiments and aspects are detailed below with particularity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a table showing times for identifying a GSM base station fordifferent measurement occasion times governed by measurement repetitionT_(meas) calculated to include measuring inter-frequency and GSM.

FIG. 1b is a table showing times for verifying a GSM base station fordifferent measurement occasion times governed by measurement repetitionT_(meas) calculated to include measuring inter-frequency and GSM.

FIG. 2a is a table similar to FIG. 1a but extended to include searchingfor an E-UTRAN neighbor cell.

FIG. 2b is a table similar to FIG. 1b but extended to include searchingfor an E-UTRAN neighbor cell.

FIG. 3 is a reproduction of section 5.2.6.1.2a of 3GPP TS 25.304 V9.3.0(2010-09)

FIG. 4 is a diagram showing dynamic selection between high priority andlow priority searching for and measuring of neighbor cells based onsignal strength received from a user equipment's serving cell accordingto an exemplary embodiment of the invention.

FIGS. 5a-e illustrate various choices made for a series of sevenmeasurement occasions based on the signal strength received from a UE'sserving cell according to exemplary embodiments of the invention.

FIG. 6 is a logic flow diagram that illustrates the operation of amethod, and a result of execution of computer program instructionsembodied on a computer readable memory, in accordance with the exemplaryembodiments of this invention.

FIG. 7 is a simplified block diagram of the UE in communication with awireless network illustrated as a base station and a RNC, which areexemplary electronic devices suitable for use in practicing theexemplary embodiments of this invention.

DETAILED DESCRIPTION

Exemplary embodiments of these teachings address the above problem byenabling a UE to dynamically switch between coverage and service basedmeasurements, thereby utilizing its cell-FACH measurement occasions moreeffectively. As quantified above, sharing the cell-FACH measurementoccasions limits the UE's mobility while in the cell-FACH state andintroducing E-UTRAN to these measurement occasions will more severelylimit that mobility. As will be seen below, these teachings enable theUE to maximize the effectiveness of the limited time available. In oneaspect of these teachings the relative importance of the different typesof measurements, whether the measurements are for coverage or forservice for example, will vary as the strength of the serving cellchanges. For example, coverage is important when the serving cell isweak to better assure an alternate cell for the UE in case signalstrength from its serving cell continues to deteriorate. Service, suchas looking for hot spot coverage to enable additional or enhanced mobileservices beyond simply cellular call coverage, is more important whensignal strength from the serving cell is stronger and basic cellularcoverage from the serving cell is not reasonably in doubt in the nearterm.

With these general principles in mind, now consider a priorityre-selection algorithm in the E-UTRAN system. By this algorithm thenetwork can prioritize either a frequency layer or a RAT over another.This means that the available measurement occasions can be used moreefficiently depending on the strength of the serving cell. When E-UTRANis supported the UE will perform measurements based on the Release 8measurement rules specified in 3GPP TS 25.304, reproduced at FIG. 3. TheRelease 99 mechanism for applying measurement occasions based on a fixedmeasurement purpose, measuring either inter-frequencies or a RAT, cantherefore be improved.

The new Release 8 priority definitions means that the measurementoccasion gaps can be used for different purposes based on the prioritiesidentified by the network. These can be applied based on the variableSprioritysearch1 or Sprioritysearch2. This mechanism for overriding theRelease 99 measurement purpose can be applied to both the Release 99inter-frequency and inter-RAT measurement occasions or just inter-RATmeasurement occasions, at the network's choosing.

FIG. 4 illustrates conceptually an exemplary embodiment of theinvention, divided into three areas of serving cell signal strength.When the serving cell strength is strong and in the higher prioritysearch region, above a second threshold which corresponds for example tothe “Sprioritysearch” parameter as shown at FIG. 4, the UE can overridethe Release 99 measurement purposes and use the measurement occasions todetect high priority RATs and/or inter-frequencies. In the specificexamples below the higher priority is the E-UTRAN system, but in anotherexemplary embodiment can be a UTRAN inter-frequency layer. So a lowerpriority neighbor cell search better aids the UE in maintaining wirelessconnectivity, while a higher priority neighbor cell search is foraccessing enhanced wireless service as compared to the serving cell,such as for example increased data rates E-UTRAN offers over WCDMA/HSPA.This connectivity/service priority distinction is true whether thesearches are inter-frequency or inter-RAT searches,

Further at FIG. 4, when the serving cell signal strength drops belowthat second threshold to lie within the “ALL priority” search regionshown there, the UE can chose to schedule the measurement occasionsbased on either GERAN or E-UTRAN based on their determined signalstrengths, or prioritize the lower priority RAT which in this case isGERAN. When the serving cell signal strength drops further to fall belowa first threshold shown in FIG. 4 as “Thresh Serving low”, coveragebecomes a more pressing concern and so the UE utilizes its measurementoccasions to search for inter-frequency and GERAN neighbor cellsaccording to conventional Release 99 measurement occasion purposes.

FIGS. 5a-e illustrate use of the UE's measurement occasions in variousinstances according to an exemplary embodiment of the invention, inwhich each figure illustrates seven consecutive measurement occasionsfor a UE. FIG. 5a illustrates the UE's conventional (Release 99measurement purpose is fixed) use of them; each alternate measurementoccasion is used for an inter-frequency search and measurement ofneighbor cells (the FDD blocks), and for an inter-RAT search andmeasurement of neighbor cells operating according to GERAN (the GERANblocks). The search pattern of FIG. 5a might also result from the UE'sserving cell signal strength lying between the first and secondthresholds of FIG. 4 (so the UE can choose which inter-RAT neighbor tomeasure).

The search pattern of FIG. 5b represents the case in which the UE'sserving cell signal strength is very good, above the second threshold ofFIG. 4 which means coverage by the serving cell is assured and themeasurement occasions are used to search for better/enhanced coverage.At FIG. 5b the UE uses its inter-RAT measurement occasions to search andmeasure neighbor E-UTRAN cells and uses its inter-frequency measurementoccasions to search and measure neighbor cells on a different frequencylayer than its serving cell. The same pattern of FIG. 5b may also resultfrom the serving cell signal strength lying between the first and secondthresholds as in FIG. 5 a.

FIG. 5c also considers the case in which the serving cell signalstrength is above the higher second threshold, but in this example theUE utilizes both its inter-frequency FDD measurement occasions and itsinter-RAT measurement occasions for high priority searching, which inthis case is for E-UTRAN neighbor cells.

FIG. 5d represents the case in which the signal strength from the UE'sserving cell is below the lower first threshold of FIG. 4, in which caseall the UE's measurement occasions are low priority searches to betterassure that basic wireless coverage is maintained. FIG. 5e showsspecifics for the low priority search occasions of FIG. 5d , which inthis case is the same as FIG. 5a and the conventional alternatingbetween inter-frequency searching and inter-RAT searching for neighborsoperating in the GERAN system.

In another embodiment there is only one threshold so that the higherpriority searches are done when the signal strength of the UE's servingcell is higher than that threshold, and the lower priority searches aredone when that signal strength is lower than the threshold.

Exemplary embodiments of these teachings exhibit the technical effect ofenabling the UE to use GSM Release 99 measurement occasions rules in analgorithm which allows the UE to also search for another higher priorityRAT and/or frequency. Conventionally, while a UE is in the CELL FACHstate it has no mobility to an E-UTRA neighbor cell and so the CELL FACHUE will stay within the UTRA system (GSM and GERAN in these examples)and will not be able to re-select to a neighbor which offers the higherdata rates that E-UTRA or some other higher priority layers might offer.Performance of re-selections to UTRA and GERAN frequencies should not beinhibited because the purpose for which the measurement occasion is usedswitches back and forth, based on the strength of the UE's serving cell.Another technical effect is that there is no change on the network sideand so these solutions are quite straightforward to implement despitethe highly structured nature of wireless cellular communications.

FIG. 6 is a logic flow diagram which describes an exemplary embodimentof the invention from the perspective of the UE. FIG. 6 may beconsidered to illustrate the operation of a method, and a result ofexecution of a computer program stored in a computer readable memory,and a specific manner in which components of an electronic device areconfigured to cause that electronic device to operate. The variousblocks shown in FIG. 6 may also be considered as a plurality of coupledlogic circuit elements constructed to carry out the associatedfunction(s), or specific result of strings of computer program codestored in a memory.

Such blocks and the functions they represent are non-limiting examples,and may be practiced in various components such as integrated circuitchips and modules, and that the exemplary embodiments of this inventionmay be realized in an apparatus that is embodied as an integratedcircuit. The integrated circuit, or circuits, may comprise circuitry (aswell as possibly firmware) for embodying at least one or more of a dataprocessor or data processors, a digital signal processor or processors,baseband circuitry and radio frequency circuitry that are configurableso as to operate in accordance with the exemplary embodiments of thisinvention.

At block 602 the signal strength of a serving cell is determined. In anembodiment that serving cell is operating according to a 1^(st) radiotechnology, which in the above examples is the WCDMA or HSPA. At block604, based on that determined signal strength a selection or choice ismade whether to utilize a measurement occasion for a lower priorityneighbor cell search or for a higher priority neighbor cell search. Thelower priority neighbor cell search is for maintaining wirelessconnectivity and the higher priority neighbor cell search is foraccessing enhanced wireless service as compared to the serving cell. Ina specific embodiment, the measurement occasion of block 604 is acell-FACH measurement occasion.

The remainder of FIG. 6 illustrates more specific implementations forblocks 602 and 604. Block 606 refers to the inter-frequency search, sothe higher priority neighbor cell search of block 604 is at block 606 asearch in a frequency layer higher in priority than the serving layer ofthe serving cell. Similarly, the lower priority neighbor cell search ofblock 604 is at block 606 a search in a frequency layer having apriority that is lower than or equal to priority of the serving layer ofthe serving cell.

Block 608 refers to the inter-RAT search, which is generally statedthere as the serving cell operating according to a first radio accesstechnology RAT, the higher priority neighbor cell search of block 604 isat block 608 a search in a third RAT, and the lower priority neighborcell search of block 604 is at block 608 a search in a second RAT.

Block 610 details the specific RATs from the above examples and also thesecond threshold from FIG. 4: the first RAT is WCDMA, the second RAT isGERAN, the third RAT is E-UTRAN, the threshold is a first threshold andthe measurement occasion is utilized for the E-UTRAN neighbor cellsearch if the determined signal strength is above a second thresholdhigher than the first threshold.

Block 612 refers to the specific inter-RAT example of FIG. 5c in whichthe measurement occasion is utilized for the E-UTRAN neighbor cellsearch (higher priority inter-RAT search) if the determined signalstrength is above the second threshold regardless of whether themeasurement occasion is an inter-RAT measurement occasion or aninter-frequency measurement occasion. Conventionally the differentmeasurement occasions themselves are not specifically allocated forinter-frequency or inter-RAT purposes; 3GPP TS 25.133 only states thatthey are to be shared equally by the modes which the UE has capabilityfor and that are in the monitored set signaled by the network. It thenfollows that for a UTRAN-specific implementation, the above concept maybe more precisely stated as seen at block 312: the cell-FACH measurementoccasion is utilized only for the E-UTRAN neighbor cell searchregardless of whether GSM (GERAN) or inter-frequency neighbour cellshave been configured for the UE which is operating under the servingcell noted at block 602.

In a specific embodiment, FIG. 6 may be considered to represent actionsof a modem which may be apart from or disposed within the above UE.

Embodiments of the invention may be implemented as an apparatus whichhas determining means and choosing means. The determining means is fordetermining signal strength of a serving cell as in block 602 of FIG. 6,and may by example be a measuring means. Specific embodiments of suchdetermining/measuring means may be for example a radio receiver and/or aprocessor. The choosing means is for choosing whether to utilize a(e.g., cell-FACH) measurement occasion for a lower priority neighborcell search or for a higher priority neighbor cell search, and as shownat block 604 of FIG. 6, this choosing means bases its choice on thesignal strength determined by the determining means. Specificembodiments of such choosing means may be for example at least oneprocessor in conjunctions with computer instructions such as analgorithm or priority measurement rules stored on a computer readablememory. As above, the lower priority neighbor cell search is formaintaining wireless connectivity and the higher priority neighbor cellsearch is for accessing enhanced wireless service as compared to theserving cell.

Reference is now made to FIG. 7 for illustrating a simplified blockdiagram of various electronic devices and apparatus that are suitablefor use in practicing the exemplary embodiments of this invention. InFIG. 7 a wireless network (base station 22 and RNC 24) is adapted forcommunication over a wireless link 21 with an apparatus, such as amobile terminal or UE 20, via a network access node such as a basestation/NodeB 22 or relay station. The network may include a radionetwork controller RNC 24, which provides connectivity with furthernetworks (e.g., a publicly switched telephone network PSTN and/or a datacommunications network/Internet).

The UE 20 includes processing means such as at least one data processor(DP) 20A, storing means such as at least one computer-readable memory(MEM) 20B storing at least one computer program (PROG) 20C,communicating means such as a transmitter TX 20D and a receiver RX 20Efor bidirectional wireless communications with the base station 22 viaone or more antennas 20. Also stored in the MEM 20B at reference number200 is the priority measurement rules, more particularly an algorithmfor choosing, based on signals strength of the serving cell 22, whetherto use a next measurement occasion for a high priority search (E-UTRANin the inter-RAT examples) or a low priority search (GERAN in theinter-RAT examples) as detailed above.

The base station 22 also includes processing means such as at least onedata processor (DP) 22A, storing means such as at least onecomputer-readable memory (MEM) 22B storing at least one computer program(PROG) 22C, and communicating means such as a transmitter TX 22D and areceiver RX 22E for bidirectional wireless communications with the UE 20via one or more antennas 22F. There is a data and/or control path 25coupling the base station 22 with the RNC 24, and another data and/orcontrol path 23 coupling the base station 22 to other base stations/nodeBs/access nodes.

Similarly, the RNC 24 includes processing means such as at least onedata processor (DP) 24A, storing means such as at least onecomputer-readable memory (MEM) 24B storing at least one computer program(PROG) 24C, and communicating means such as a modem 24H forbidirectional wireless communications with the base station 22 via thedata/control path 25. While not particularly illustrated for the UE 20or base station 22, those devices are also assumed to include as part oftheir wireless communicating means a modem which may be inbuilt on an RFfront end chip within those devices 20, 22 and which also carries the TX20D/22D and the RX 20E/22E.

At least one of the PROGs 20C/20G in the UE 20 is assumed to includeprogram instructions that, when executed by the associated DP 20A,enable the device to operate in accordance with the exemplaryembodiments of this invention, as detailed above. The base station 22may also have software stored in its MEM 22B to implement certainaspects of these teachings as detailed above, s as to know or betteranticipate how the UE 20 will utilize its measurement occasions. In thisregard the exemplary embodiments of this invention may be implemented atleast in part by computer software stored on the MEM 20B, 22B which isexecutable by the DP 20A of the UE 20 and/or by the DP 22A of the basestation 22, or by hardware, or by a combination of tangibly storedsoftware and hardware (and tangibly stored firmware). Electronic devicesimplementing these aspects of the invention need not be the entire UE 20or base station 22, but exemplary embodiments may be implemented by oneor more components of same such as the above described tangibly storedsoftware, hardware, firmware and DP, modem, system on a chip SOC or anapplication specific integrated circuit ASIC.

In general, the various embodiments of the UE 20 can include, but arenot limited to personal portable digital devices having wirelesscommunication capabilities, including but not limited to cellulartelephones, navigation devices, laptop/palmtop/tablet computers, digitalcameras and music devices, and Internet appliances.

Various embodiments of the computer readable MEMs 20B and 22B includeany data storage technology type which is suitable to the localtechnical environment, including but not limited to semiconductor basedmemory devices, magnetic memory devices and systems, optical memorydevices and systems, fixed memory, removable memory, disc memory, flashmemory, DRAM, SRAM, EEPROM and the like. Various embodiments of the DPs20A and 22A include but are not limited to general purpose computers,special purpose computers, microprocessors, digital signal processors(DSPs) and multi-core processors.

Various modifications and adaptations to the foregoing exemplaryembodiments of this invention may become apparent to those skilled inthe relevant arts in view of the foregoing description. While theexemplary embodiments have been described above in the context of theUTRAN Release 99 system, it should be appreciated that the exemplaryembodiments of this invention are not limited for use with only this oneparticular type of wireless communication system, and that they may beused to advantage in other wireless communication systems.

Further, some of the various features of the above non-limitingembodiments may be used to advantage without the corresponding use ofother described features. The foregoing description should therefore beconsidered as merely illustrative of the principles, teachings andexemplary embodiments of this invention, and not in limitation thereof.

1. An apparatus, comprising: at least one processor; and at least onenon-transitory memory storing a computer program; in which the at leastone non-transitory memory with the computer program is configured withthe at least one processor to cause the apparatus to at least: determinesignal strength of a serving cell; and choose, based on the determinedsignal strength, whether to utilize a cell forward access channelmeasurement occasion for a lower priority neighbor cell search or for ahigher priority neighbor cell search, in which the lower priorityneighbor cell search is for maintaining wireless connectivity and thehigher priority neighbor cell search is for accessing enhanced wirelessservice as compared to the serving cell, wherein: the serving celloperates according to a first radio access technology (RAT) which isWCDMA.
 2. The apparatus according to claim 1, wherein the higherpriority neighbor cell search comprises a search in a third RAT which isE-UTRAN or in a frequency layer higher in priority than a serving layerof the serving cell.
 3. The apparatus according to claim 1, wherein thelower priority neighbor cell search comprises a search in a second RATwhich is GERAN or in a frequency layer having a priority that is lowerthan or equal to priority of the serving layer of the serving cell. 4.The apparatus according to claim 1, wherein the measurement occasion isutilized for the lower priority neighbor cell search when the determinedsignal strength is below a threshold.